The production of single-crystal components is becoming increasingly important in order to allow ever further increases in performance to be achieved, in particular in the field of turbines. Blades and vanes of turbines, such as aircraft engine turbines or gas turbines for generating energy in power plants, are exposed to high loads. As the performance, efficiency and emissions of gas turbines continue to improve, the thermal and therefore mechanical boundary conditions imposed on turbine guide vanes in the first stage(s) are becoming increasingly extreme. Particular loads occur, for example, as a result of the platforms being made increasingly thin, as a result of a changing working medium and as a result of reduced external cooling on account of cooling leaps in the passage edge region of the gas turbine. A further improvement in the performance in terms of materials properties is required, since to reduce the consumption of cooling air in the gas turbine, the number of vanes in the ring needs to be reduced in favor of larger-sized vanes. To make it possible to cope with these boundary conditions, it is desirable to change material or to change material structure towards single crystals, as in part already being attempted in the rotor blade area of gas turbines. Components which are formed from single crystals only have grain boundaries which do not cause any significant weak points in the material compared to the material as a whole as temperatures rise.
The production of single-crystal components has already been known for many years. To do this, first of all molten material is poured into a casting mold. The casting mold has the negative shape structure of the component which is to be produced and is therefore also referred to as a negative mold. When the molten material solidifies in the negative mold, it acquires the desired shape structure of the component. The shape structure generally comprises a plurality of regions, known as structural parts. The casting mold holding the molten material is located inside a furnace, so that the molten material is heated further and remains liquid. To produce single crystals, the negative mold is slowly moved out of the furnace, so that the solidification process begins outside the furnace and the solidification front follows the negative temperature gradient. To provide the single crystal with the desired orientation, a cochleate tip of the casting mold, filled with molten material, is initially solidified from the outermost tip. The cochleate tip is referred to as the selection helix. The helix is used to select the crystal growth direction. The single crystal therefore forms the incipient solidification front and in the desired case continues to grow through the entire negative mold. The negative mold has to be passed through the negative temperature gradient at a speed which is matched to the crystallization rate of the metal. If the molten material is passed through the negative temperature gradient more quickly than the crystal growth rate, new creation occurs and additional crystals which have a different orientation than the desired single crystal are formed. If this occurs, the component is faulty and can no longer be used. In particular the production of single crystal turbine guide vanes with greatly overhanging platforms is very difficult. The turbine guide vanes comprise a plurality of structural parts which form the shape structure of the component. A particular problem is the transition from a platform to a profiled-section part and vice versa. The platform forms the region of the guide vane which in the turbine lies parallel to and flush with the inner radial surface of the guide vane carrier, while the profiled-section parts are firstly the main vane section and secondly the connecting element between the guide vane carrier and the guide vane. A problem with this arrangement is that the plane normal to the platform is virtually perpendicular to the plane normal to the profile. The growth direction of the single crystal is generally selected in such a way that it runs substantially along a longitudinal axis of the profile-section part and parallel to the plane normal to the platform. The planes are almost substantially perpendicular to one another and extend in different directions. Accordingly, it is necessary to realize a virtually instantaneous change in the solidification front surface from the small surface of the profiled-section part to the substantially wider surface of the platform. In this region, it is necessary to respond with a drop in the rate at which the casting mold in the furnace is lowered from the heated region into the unheated region, or to provide a corresponding number of grain-maintaining means in the form of a diversion from the profiled-section part into the outer platform regions. The effects of both options are very difficult to predict and moreover are subject to fluctuations in the process, so that the discharge rate or productivity drops. Hitherto, it has been attempted to solve this problem by searching for an optimum orientation with respect to the negative temperature gradient for the platform. By tilting the casting mold in the furnace in order to optimize the orientation of the platform with respect to the negative temperature gradient, the problem is improved in one direction, since the solidification front then fans open along a ramp into the platform, so that the change in cross-section is attenuated. Despite this, a right-angled connection still remains. A further problem is that of bringing the single crystal which has fanned out in the platform back together into a narrower structural part of the guide vane, which once again presents the problem of the right-angled connection with a spontaneous change in the surface of the solidification front.
These and similar problems also manifest themselves in numerous instances for other components of other shape structures.